A number of pressure sensors are known to utilize a silicon diaphragm which deflects in response to pressure. Deflection of the diaphragm is generally detected by electronic sensing elements such as piezoresistive elements placed on the edges of the diaphragm. These sensors are generally designed so that batch fabrication is possible. The range of pressure detection will depend on the size, thickness and span of the diaphragms. One such sensor is described in U.S. Pat. No. 4,949,581. The sensor there described includes overpressure protection features. Pressure applied to the sensor is limited by two isolator diaphragms separate from the sensor itself. When a preselected differential pressure limit is exceeded, the deflection of one of the isolator diaphragms (responding to the greater pressure) bottoms against a diaphragm support. Once bottomed against the support, no further increases in pressure are transmitted to the sensor. The pressure limits of these devices are set to protect the sensor diaphragm which has a relatively low pressure limit from pressures which will permanently deform it and thereby degrade the sensor's performance. Characteristics of this system which detract from its performance include both hysteresis and diminished sensitivity as a result of the circuitous pressure paths between the overpressure protecting diaphragms and the separate sensing diaphragms.
An alternate design for a bidirectional pressure sensor is disclosed in U.S. Pat. No. 4,905,575 issued to Knecht et al on Mar. 6, 1990. According to the teachings of Knecht et al, a silicon diaphragm is mounted between two glass base plates which have recesses formed therein to receive the diaphragm and provide support across the diaphragm under overpressure conditions. The support plates serve as positive stops when the diaphragm is subject to overpressure and thus prevent overstressing the diaphragm. The pressure sensor disclosed in this patent further includes a diaphragm having grooves formed on opposite surfaces to define a center deflecting portion. The grooves provide a "free edge" effect which reduces bending stress at the diaphragm edge and permit a higher operating pressure without breakage.
Removing material to provide grooves on opposite surfaces of the diaphragm, however, requires tight control tolerances during manufacture. Precise alignment of the glass base supports during assembly is also critical, especially when the sensor has an array of sensing diaphragms with different sensing ranges. Further, glass and silicon differ in strength and thermal coefficients. When the sensor is intended for applications over wide temperature and pressure ranges, the material property mismatch can create stresses and large sensing errors which may be difficult to overcome.
It is a primary object of this invention to provide a capacitive, differential, low pressure sensor which can be fabricated using semiconductor techniques and which also includes pressure overrange protection.
It is another object of this invention to provide a bidirectional, capacitive, differential low pressure sensor including bidirectional pressure overrange protection.